1. Field of the Invention
The present invention relates to a switching power supply, and particularly to a secondary switch controller adapted to derive an output voltage which is different from an input voltage.
2. The Prior Art
Secondary switch controllers of the type to which the present invention belongs are typically employed for generating a controlled DC voltage from an uncontrolled DC voltage. In a special application, the output DC voltage is higher than the input DC voltage. The circuit employs an inductance and a switching transistor connected in series, with the switching transistor being alternately fully driven and cut off. The output voltage is made available at a tap between the inductance and the switching transistor, and is connected through a diode to a smoothing capacitor.
As described in Tietze and Schenk, Halbleiterschaltungstechnik, 5th Edition, Chapter 16.5.1 a switching transistor may be operated with a constant frequency drive signal. The pulse duty factor of the drive signal is controlled, i.e., the ratio of the on time to the frequency, so that the output voltage corresponds to a reference voltage. The disadvantage of this circuit is that a reference voltage is required, and a control circuit having a fixed frequency is also required. As a result of these requirements, relatively large switching losses can occur.